Embodiments of the present invention generally relate to ultrasound breast screening systems, and more particularly to ultrasound breast screening systems having automated ultrasound transducers positioned on at least one compression plate.
Typically, x-ray mammography is used as the primary screening procedure for detection of breast lesions. For each x-ray mammogram screening of a patient, a top view, referred to as a cranio-caudal view (“CC” view), and a mediolateral oblique view (“MLO” view) are usually taken.
X-ray mammography, however, poses various patient comfort issues. For example, a patient's breast is typically compressed during a mammographic procedure. The force of the compression, and the orientation of the compressing members, may cause pain and overall discomfort. Additionally, x-ray mammography may be hazardous due to the fact that x-ray mammography uses ionizing radiation. Further, studies have shown that mammography generates false positives in more than 10% of patients and that that x-ray mammography is not always effective and accurate with respect to dense breasted women because lesions masked by dense breast tissue may go undetected.
Sonography, or ultrasound, has been used as a complementary screening procedure to confirm screening results. In fact, sonography has gained acceptance as a viable alternative to x-ray mammography for breast imaging, due to the drawbacks and hazards associated with x-ray mammography. For example, sonography has been used when X-ray mammography has failed to confirm the results of a manual examination.
When sonography is used in conjunction with x-ray mammography, the rate of accurate detection of lesions improves to over 90%. However, two separate imaging procedures, that is, x-ray mammography and ultrasound breast imaging, are required for a single patient, which is inconvenient and may even delay diagnosis. Further, the dual use of x-ray mammography and ultrasound breast imaging requires skilled specialists and typically at least twenty minutes of screening time.
Hand-held ultrasound transducer probes have been used in examinations to complement X-ray mammography. A drawback of such freehand examinations, when used to supplement mammography, is the inability to provide geometric registration between the mammogram and ultrasound images. The lack of registration makes it difficult to relate what is seen in the ultrasound image to what is seen in the mammogram. Furthermore, the three dimensional shape of the lesions and the increased vascularity associated with carcinoma make volumetric spatial registration of the ultrasonic data with a mammogram desirable.
U.S. Pat. No. 5,479,927, issued to Shmulewitz, entitled “Methods and Apparatus for Performing Sonomammography and Enhanced X-Ray Imaging,” (the “'927 patent”) which is hereby expressly incorporated herein in its entirety, describes a system that combines mammography equipment with an ultrasound transducer to generate ultrasonic images of the internal structure of breast tissue that are in geometric registration with a mammographic image. The system disclosed in the '927 patent includes a radiolucent and sonolucent compression plate. Either before or after the x-ray exposure, a carriage-mounted ultrasound transducer is translated in increments across the compression plate to generate a plurality of sectional views of the breast tissue. The x-ray and ultrasound images produced by this sonomammography apparatus are ideally in geometric registration. Those images may in turn be processed by a conventional microprocessor-based workstation to provide holographic views of the internal features of a patient's breast.
X-ray mammography images are typically obtained using a plastic plate to compress the breast. The compression plates used in x-ray mammography were historically made of polycarbonates, which are acoustically opaque, because of their tensile strength and transparency to x-ray. Most other materials potentially useful for the compression plates in mammography equipment have relatively high densities and thus exhibit relatively high attenuation and reflection coefficients for acoustic wave energy. The '927 patent discloses a compression plate made of a high-performance acoustically transparent (sonolucent) and x-ray transparent (radiolucent) film that is sufficiently rigid to serve as a compression plate at a thickness of about 25 micron (1 mil).
As shown in the '927 patent, however, a breast is compressed between two compressive members that are parallel with one another, and typically parallel with the plane of the floor. The compressive members move toward each other to compress the breast. The breast typically needs to be substantially flattened between the plates so that the plates may be in proper contact with the breast for imaging. A certain force, which may vary among patients, is used to substantially flatten the breast between the two parallel plates so that proper contact is obtained with substantially the entire breast. However, the force needed to properly flatten the breast often causes the patient pain and discomfort.
U.S. patent application No. 2003/0007598, filed May 31, 2002, entitled “Breast Cancer Screening With Adjunctive Ultrasound Mammography,” (the “'598 application”) which is hereby expressly incorporated herein in its entirety, discloses systems and methods for intuitive viewing of adjunctive ultrasound data concurrently with x-ray mammogram information. Instead of registering the ultrasound images with the x-ray images, the '598 application teaches displaying “thick” slice images near an x-ray mammogram so that a screening radiologist may quickly view the thick slice images for assistance in interpreting the x-ray mammogram.
U.S. application No. 2002/0173722, filed Apr. 5, 2001, entitled “Focus Correction for Ultrasound Imaging Through Mammography Compression Plate,” (the “'722 application”), which is hereby expressly incorporated herein in its entirety, describes an ultrasound imaging system capable of acquiring an image of a tissue through a plastic plate. The '722 application discloses a beamformer that is programmed with pre-stored transmit and receive time delays that are computed to correct the effects of refraction caused by an intervening plastic mammography compression plate of an x-ray mammography system. The correction enables acquisition of an in-focus ultrasound image taken under the same conditions as an x-ray mammography compression image. As disclosed in the '722 application, because the ultrasound and x-ray mammography images are formed from the same source under the same conditions, the images may be registered.
Conventional ultrasound imaging systems comprise an array of ultrasonic transducer elements that transmit an ultrasound beam and receive the reflected beam from the object being studied. After a focused ultrasound wave is transmitted, the system switches to receive mode after a short time interval, and the reflected ultrasound wave is received, beamformed and processed for display. Typically, transmission and reception are focused in the same direction during each measurement to acquire data from a series of points along an acoustic beam or scan line. The receiver is dynamically focused at a succession of ranges along the scan line as the reflected ultrasound waves are received.
An ultrasound array typically has a plurality of transducer elements arranged in one or more rows. The elements are usually driven with separate voltages. By selecting the time delay (or phase) and amplitude of the applied voltages, the individual transducer elements in a given row may be controlled to produce ultrasonic waves that combine to form a net ultrasonic wave that travels along a preferred beam vector direction and is focused at a selected point along the beam. The beamforming parameters of each of the firings may be varied to provide a change in maximum focus or otherwise change the content of the received data for each firing, for example, by transmitting successive beams along the same scan line with the focal point of each beam being shifted relative to the focal point of the previous beam. For a steered array, by changing the time delays and amplitudes of the applied voltages, the beam with its focal point may be moved in a plane to scan the object.
The same principles apply when the transducer probe is employed to receive the reflected sound in a receive mode. The voltages produced at the receiving transducer elements are summed so that the net signal is indicative of the ultrasound energy reflected from the object. As with the transmission mode, the focused reception of the ultrasonic energy is achieved by imparting separate time delay (and/or phase shifts) and gains to the signal from each receiving transducer element.
FIG. 1 illustrates a conventional sector array 10 that may be used with an ultrasound probe. For the sake of clarity, the ultrasound probe is not shown. Rather, only the sector array 10 and field of view 12 are shown. The sector array 10 includes a plurality of ultrasound elements 14. As shown in FIG. 1, the sector array 10 transmits and receives ultrasound waves over a wide field of view 12 by applying appropriate time delay to steer the ultrasound beam. The width A of the field of view 12 is wider than that of a linear array, as shown below with respect to FIG. 2. However, the imaging resolution of the sector array 10 decreases with increased depth in the direction of line B.
FIG. 2 illustrates a conventional linear array 16 that may be used with an ultrasound probe. Similar to the sector array 10, the linear array 16 includes a plurality of ultrasound elements 18. As shown in FIG. 2, the linear array 16 transmits and receives ultrasound waves over a relatively narrow field of view 20, as compared to that of the sector array 10 due to limited steering capabilities of linear probes. That is, the width C of the field of view 20 of the linear array is not as wide as the width A of the field of view 12 of the sector array 10, as shown in FIG. 1. However, while the linear array 16 exhibits a relatively narrow field of view 20, the imaging resolution of the linear array 16 is uniform throughout the field of view 20.
FIG. 3 illustrates a conventional curved array 22 that may be used with an ultrasound probe. The curved array 22 is defined by a plurality of ultrasound elements 24. Similar to the linear array 16 shown in FIG. 2, the curved array 22 has limited steering capabilities. A wider field of view is obtained by shaping the array in a curved format. The curved array 22 is a hybrid of the sector array 10 and the linear array 16 in that it is designed to transmit and receive ultrasound waves over a wider field of view 26, as compared to the liner array 16, while maintaining a more uniform imaging resolution throughout the field of view 26 as compared to the sector array 10. The width D of the field of view 12 of the curved array 22 is wider than that of the linear array 16.
In conventional ultrasound probes, such as linear, sector and curved array probes, when an ultrasound beam is electronically steered off center, the ultrasound beam tends to widen. The corresponding reflected ultrasound beam reflects off an area of such a size that the data is typically “volume averaged” in order to construct an image. However, volume averaging may mask structures within a piece of anatomy, due to the fact that the image includes, in effect, estimates of the anatomical structure.
Conventional ultrasound probes, having sector, linear or curved arrays, use a single row of transducer elements, as discussed above with respect to FIG. 1-3. As is well known, using a single row of elements limits the focusing ability of the transducer elements in the near and far fields. Consequently, pathologies may be masked due to volume averaging techniques required to focus in the near and far fields.
Thus, a need exists for a more patient-friendly ultrasound breast imaging system. A need also exists for an ultrasound breast imaging system that automatically scans a patient's breast with more clarity and accuracy.